I noticed someone else posting the Drengin sending them messages, but weren't showing up in their Diplomacy Screen. I was at war with and took the last planet of the Drengin, and -- as they had ever since I took out their shipyard -- asked for peace every turn, even after I took their last planet and they no longer show up on the Diplomacy screen as a civilization.
Drengin are ugly enough as it is ... do we really need zombie Drengin? 
(sorry, couldn't resist ...)